Transmit receive switch with high power protection

ABSTRACT

Microwave switch (MS) comprising an antenna branch (AB), a receive branch (RB), and a transmit branch (TB). According to one embodiment, the switch comprises a protection circuit (CPL, DTC, CTRL), which upon a high power signal (Dtc_h) being present on the antenna branch effectuates a biasing of semiconductors on both the receive branch (RB) and the transmit branch (TB). According to one embodiment PIN diodes are provided for controlling switching between the transmit and the receiv branch as well as enabling a protective mode, the control being accomplished by selectively biasing respective diodes.

FIELD OF THE INVENTION

The present invention relates to a switch arrangement for a microwavetransmit and receive system. More particularly the invention relates toa switch and protection arrangement for a transmit and receive systemand in particular a radar system.

BACKGROUND OF THE INVENTION

In typical synthetic aperture radar systems a plurality of antennaelements are provided, through which both high power signals are emittedand corresponding low power echoes are received. So-called transmitreceive (T/R) modules have the function of switching between incomingand outgoing signals and to provide input and output phase adjustment aswell as input amplification.

It is widely known to use ferrite circulators in order to separateincoming and outgoing signals. However, circulators are only effectiveup to certain magnitudes of power.

Another drawback with ferrite circulators is their bulk and weight.

It is known to use passive limiters for protecting the input section ofa radar system.

Prior art document EP-A-0391326 discloses a receiving transmission linecomprising an antenna input post, over which signals are transmitted toa receiver. The transmission line comprises two PIN diodes. The PINdiodes are adapted to be selectively biased for reflecting signalsentering the transmission line and thereby protecting the input stageand adapted for being selectively unbiased for rendering theparallel-coupled PIN diodes yielding a high impedance and therebyopening the transmission line for signals to be received. Protection isaccomplished also when the equipment is switched off.

Prior art document U.S. Pat. No. 4,232,278 shows a radar receiverprotector circuit having a transmission line input stage comprisingfirst, second and third parallel coupled PIN diodes. The circuitdiscloses sensitivity time control, functioning as a passive limiterduring transmit and controlled attenuator during receive.

Prior art document U.S. Pat. No. 4,789,846 shows a microwavesemiconductor switch, for instance for switching an antenna betweentransmission and reception modes wherein first and second field effecttransistors and first, second and third input/output microstrip linesare integrally formed on a semiconductor substrate. The first fieldeffect transistor is connected in series between the second input/outputline and a junction point of the first second and third input/outputlines at a position spaced approximately a quarter of the wavelengthfrom the junction point. The second field effect transistor is connectedat a second position spaced approximately a quarter of the wavelengthfrom the junction point between the second position and the ground. Thetransmission paths for microwaves are switched by varying a bias voltageapplied to the gate electrodes of the field effect transistors. Theswitch offers no protection against incoming high-energy signals.

SUMMARY OF THE INVENTION

The invention seeks to accomplish a microwave switch for switchingbetween incoming signals from an antenna and outgoing signals to theantenna, while offering high power protection and being adapted forsingle chip Microwave Monolithic Integrated Circuit (MMIC) design.

This object has been accomplished by the subject matter set forth inclaim 1.

It is moreover an object to accomplish a switch with protection forincoming signals which functions disregarding the state of the switch.

These objects have been accomplished by the subject matter defined inclaim 2.

It is another object to set forth a transmit and receive module, whichcan withstand enhanced high power signals.

This object has been accomplished by claim 3.

It is a further object to accomplish still further enhanced protectionagainst incoming signals.

This object has been achieved by the subject matter defined in claim 4.

It is a further object to accomplish a microwave switch that performsrapid passive protection and effective long term protection.

This object has bee achieved by the subject matter set forth in claim 6.

Further advantages will appear from the remaining claims and thefollowing detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first preferred embodiment of the invention,

FIG. 2 shows a second preferred embodiment of the invention,

FIG. 3 shows an active and a passive characteristic of the firstpreferred embodiment of the invention for a receive branch,

FIG. 4 shows an exemplary high power pulse, and

FIG. 5 shows an active and a passive characteristic of the firstpreferred embodiment of the invention for a transmit branch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, a first embodiment of the microwave switch apparatusaccording to the invention has been shown.

The microwave switching apparatus MS comprises an antenna terminal ANT,which is adapted to be coupled to an external antenna (not shown). Themicrowave switching apparatus is also adapted to be coupled to a radarsystem (not shown), which controls the switching apparatus. The antennaterminal is being coupled over an optional capacitor C5 to an antennabranch AB, which is connected to a first junction J. The antenna branchmay advantageously comprise a coupler CPL. The first junction J couplesto a receive branch RB and a transmit branch TB.

The first junction J connects to first receive branch capacitor C3,which couples to a receive branch RB comprising a receive junction R.The receive branch RB couples to a second receive branch capacitor C1coupling to a receive amplifier RX. The receive junction R is preferablyarranged at a distance corresponding to a quarter wavelength of apredetermined wavelength from the first junction J. At the receivejunction R, a first diode D1 branches off to ground.

The first junction J connects also to first transmit branch capacitorC4, which couples to a transmit branch TB comprising a transmit junctionT. The transmit branch TB couples to a second transmit branch capacitorC2 coupling to a transmit amplifier TX. The transmit junction T ispreferably arranged at a distance corresponding to a quarter wavelengthof a predetermined wavelength from the first junction J. At the transmitjunction T, a second diode D2 branches off to ground.

The first and second diodes D1, D2 are controlled by a control circuitCTRL, which controls appropriate DC bias levels over the respectivefirst and second diode over respective logic bias lines BL over switchesS1-S4. The control circuit CTRL has a receive mode input and a transmitmode input from which logic control signals RX_on, TX_on is input fromthe radar system control logic (not shown). Moreover, the controlcircuit CTRL is connected to a detector circuit DTC, which again isconnected to coupler CPL.

If a high power signal is coming in on the antenna branch AB, the signalis transmitted over the coupler and is detected by detector DTC, whichoutputs a DC voltage signal DTC_h to control circuit CTRL.

The purpose of the coupler CPL is to pick up a small amount of theincoming signal, typically −20 dB (1%) with minimum insertion loss.Preferably, the coupler is directional, whereby only incoming signals,not outgoing signals from the transmitter, are picked up.

The detector DTC converts the RF-signal from coupler CPL to DC voltagesignal Dtc_h. Advantageously; the detector element is a high frequencySchottky diode.

The control circuit CTRL is designed to deliver bias currents (a few mA)or a reverse voltage to the diodes D1 and D2 depending on the controlsignals RX_on, TX_on and the DC voltage signal from DTC.

Advantageously, the first and second diodes are PIN diodes that are ableto withstand high power transients and therefore are suitable inhigh-power circuits.

Advantageously, the transmitter, receiver and the antenna are terminatedinto 50 ohms.

For the receive branch there is provided a first switch S1 for couplingthe ground and a second switch S2 for coupling a current source 12 overinductor L1 to receive junction R.

For the transmit branch there is provided a third switch S3 for couplinga current source 13 and a fourth switch S4 for coupling to voltage −U4over inductor L2 to transmit junction T.

Switches S1-S4 are controlled by control unit CTRL.

The functionality of the circuit shall now be explained with referenceto table 1 below. For signals RX-on, TX-on, Dtc_h, one designates a highbiasing voltage and zero designates a low or ground biasing voltage. Forswitches S1-S2, zero indicates an open switch while one indicates aclosed switch. x indicates that either a 0 state or 1 state can beadopted. The table below illustrates the permanent states after a statetransition and not the transitional conditions. TABLE 1 Mode RX-on TX-onDtc_h S1/GND S2/I2 S3/I3 S4/−U4 transm 0 1 0 0 1 0 1 receive 1 0 0 1 0 10 incident x x 1 0 1 1 0 protect 0 0 x 0 1 1 0

A forward current through the respective diode D1 or D2 renders thediode conductive causing the diode to present a low impedance value overits terminals, while a low or reverse biasing voltage 0 over the dioderenders the diode non-conducting causing it to present a high impedanceover its terminals.

As appears from the table above, when the radar system adapts a receivemode, a signal RX_on is present on control circuit CTRL, switch S1connects to ground and switch S2 is open. Thereby, input signals derivedfrom the antenna terminal (ANT) are received over the receive branch RB.A DC return path is established through switch S1 and coil L1, whichallows the rectified current in D1 stemming from the incident signal toflow through the diode rendering the diode self biased if exposed toincident high power.

In receive mode, switches S3 and S4 are operated so as to feed a current13 through coil L2 into transmit branch T and through diode D2, forcingdiode D2 into a low impedance state. Thereby, transmit branch TB isgrounded at point T, whereby signals entering the transmit branch arereflected by the shortcut. The impedance seen at junction J into thetransmit branch is very high, since the distance between junctions J andT corresponds approximately to a quarter wavelength of the intendedsignal.

In transmit mode switches S1 and S2 are operated so as to drive acurrent I2 through coil L1 into branch RB and through diode D1, wherebythe forward biasing current forces diode D1 into a low impedance state.Signals which enter the receive branch RB are thereby reflected by theshortcut. The distance between junctions J and R corresponding to aquarter wavelength of the intended signal renders the impedance seenfrom the junction J towards the receive branch RB very high. Thereby,the transmit signal can pass without attenuation and the receive branchis protected.

To avoid interference between transmission and power protection, it issuitable to back-bias diode D2 in the transmit branch duringtransmission. Therefore, switches S3 and S4 are operated to feed anegative voltage U4 over coil L2 and diode D2.

Should an incident high power microwave signal be received on theantenna branch, such microwave signal propagates via coupler CPL todetector DTC. A signal DTC_h is provided to the control circuit CTRL.When control circuit CTRL detects the DTC_h signal, it causes bothdiodes D1 and D2 to be biased, disregarding the state of the signalsRX_on and TX_on, leading to the incoming high power microwave signalbeing reflected from the antenna terminal ANT. Thereby, the deviceadapts a protective mode.

A protective mode may also be actively controlled from the controlcircuit as indicated in the table 1 above.

Since the signalling paths from the coupler CPL and through the controlcircuit CTRL to the respective diodes D1 and D2 are not insignificantand since it would typically be infeasible to render the transmit branchand receive branch sufficiently long to cause appropriate delays, thecontrol circuit CTRL can not accomplish an active switching of thediodes before the incoming high power pulse reaches the diodes.

However, according to the invention, passive protection is accomplishedby the diodes D1 and D2 in an initial phase of an incoming a high-energyimpulse, when in receive or transmit mode. The passive protection is dueto the self-biasing effect of the diodes if subject to an incoming highpower microwave signal, whereby the diodes represents a zero impedance,which causes the incoming wave to be reflected at the respective diode.

In FIG. 3, the dampening properties of the circuit have been shown for apassive mode P and an active mode A, whereby the power at input RX isshown as a function of the incoming impulse P_ant at antenna input ANT.The characteristic P corresponds to a situation where the protectioncircuit-coupler CPL, detector DTC, control circuit CTRL, switchesS1-S4—would not be provided. Hence, the passive mode corresponds to thesituation where the respective diodes are not biased when a high-energyimpulse is arriving at the antenna branch. The active characteristiccorresponds to the dampening characteristic should the diode already bein a biased state when a high power pulse impinges on the antennabranch.

The passive protection also protects the receiver and the transmitterwhen the coupler/control circuitry is non-operational, either as aneffect of a fail state or as an affect of that the system is not in use(power off).

As appears from FIG. 3, the dampening properties of the activelycontrolled diodes are more effective than the passive dampening due tothe current which keeps the diode in a low impedance state during thefull radio frequency state.

It should be noted that the dampening properties at input TX has asimilar shape to FIG. 3. However, the active curve is controlled by thelevel of the reverse voltage U as illusrated in FIG. 5. The protectionis typically designed so that in passive mode the limit level isslightly higher than the transmit power.

In FIG. 4, the positive envelope of an exemplary high frequency highpower pulse train has been shown. The pulse train for instance has aduration of about 1 micro second and within 100 ns 90% of the peak poweris reached. Off course, the incident signal could have many othershapes. It is an object of the invention to protect against unwantedpulses, such as jamming or other inadvertent signals. In this context, aRF signal with a power level exceeding the maximum allowed input powerto the receiver RX and/or the transmitter TX is considered a high powersignal. Maximum input power is typically rated to about +15 dBm for manyreceivers.

When exposed to the above exemplary high power pulse train when inreceive or in transmit mode, the switch according to the inventiondampens the incoming pulse according to the passive characteristic p upuntil time t1, denoted in FIGS. 3 and 4. Subsequently, the controlcircuit effects the switching at t2, allowing for the more effectivedampening according to the active characteristic a.

It is noted, that the control circuit CTRL is able to bias (switch on)the PIN diodes in time, if the sum of the delays in the detector, thecontrol circuit and the PIN diodes is less than the rise time of theincoming high power pulse. However, if this is not the case, the energyis still limited since the exposure to high power pulses is of limitedduration as appears from the transition from t1 to t2. It is noted thatcomponent failure could be caused by break down due to a peak effect orbum off due to a total energy exposure.

Thus it is possible to use the invention without the control circuit andto take advantage of the passive limitation in the diodes, but it woulddecrease the ability to handle the highest incident power pulses.

In FIG. 2, a further embodiment of the invention has been shown. As inFIG. 1, switches S1-S4, inductors L and control units CTRL are providedand these have the same functionality.

According to this embodiment, further diodes D3 and D4 have beeninserted into the transmit branch and the receive branch, at points Cand D, respectively. Moreover, a fifth diode D5 in series with aresistor R1 connects to the first junction J and is biased over biasline BLJ from control circuit CTRL. The above diodes D3 and D4 arearranged at a quarter wavelength from the first and second diode,respectively.

The further diode D3 in the receive branch obviates leakage power toreach the receiver.

The further diode D4 in the transmit branch is coupled to resistor R2which presents a load to the transmitter in the protected mode.

The fifth diode D5 loads the antenna when in the protective mode. Thefifth diode D5 is arranged at the junction, J and couples throughresistor R1 to ground. Switches S5 and S6 selectively provide a biasingcurrent I5 or a reverse voltage U6, to junction J.

The circuit operates in the same manner as the circuit shown in FIG. 1when in receive or transmit mode. When a high level signal is present onthe antenna branch, the fifth diode is biased, as shown in the tablebelow, and an incoming signal is, apart from being reflected at diodesD1 and D2, absorbed in resistor R2. Residual power is moreover reflectedat diodes D3 and D4.

The main improvement is to achieve higher isolation between the antennaand the receiver/transmitter, and that the transmitter and antenna areterminated during protection or high power modes. TABLE 2 S1/ S2/ S3/S4/ S5/ S6/ Mode RX-on TX-on Dtc_h GND I1 I3 −U4 I5 −U6 trans. 0 1 0 0 10 1 0 1 receive 1 0 0 1 0 1 0 0 1 incident x x 1 0 1 1 0 1 0 protect 0 0x 0 1 1 0 1 0

1-12. (canceled)
 13. A microwave switch comprising: an antenna branchcoupled to a first junction, the first junction coupled to a receivebranch coupled to a receiver; the receive branch comprising a firstsemiconductor which can be selectively biased in order to reflectsignals transmitted over the receive branch wherein the junction iscoupled to a transmit branch coupled to a transmitter; the transmitbranch comprising a second semiconductor which can be selectively biasedin order to reflect signals transmitted over the transmit branch; acontrol circuit connected to the receive branch by a first bias line andconnected to the transmit branch by a second bias line, the controlcircuit, when operating in a receive mode, biasing the semiconductor inthe transmit branch, and when operating in a transmit mode biasing thesemiconductor in the receive branch; and, a protection circuit which,upon a high power signal being present on the antenna branch,effectuates a biasing of semiconductors on both the receive branch andthe transmit branch.
 14. The microwave switch according to claim 13,wherein the first and second semiconductors are diodes that areself-biased if subject to a high power signal on the antenna input. 15.The microwave switch according to claim 13, wherein the switch comprisesa diode in series with a resistor which is coupled to the first junctionand is biased by the protection circuit if a high power signal ispresent on the antenna branch.
 16. The microwave switch according toclaim 13 wherein the protection circuit comprises: a coupler coupling tothe antenna branch; and a detector for generating a DC signal uponreception of a high power signal on the coupler; wherein the controlcircuit feeds a bias signal on the transmit branch or the receivebranch.
 17. The microwave switch according to claim 13 wherein, uponreception of a high power signal on the antenna branch, the switchinitially adapts a passive mode wherein diodes are biased by theincoming signal and subsequently adapts an active mode in which theprotection and control circuit biases the diodes.
 18. The microwaveswitch according to claim 16, wherein the coupler is directional suchthat substantially only incoming signals are picked up.
 19. Themicrowave switch according to claim 13, wherein the control circuit isoperated in correspondence with modes of a radar system.
 20. Themicrowave switch according to claim 13, wherein a protection mode isprovided in which all diodes are biased.
 21. The microwave switchaccording to claim 13, wherein the receive branch is coupled torespective switches operable by the control circuit for feeding abiasing current or a ground level potential over the diode or diodes inthe receive branch.
 22. The microwave switch according to claim 13,wherein the transmit branch is coupled to respective switches operableby the control circuit for feeding a biasing current or a reversevoltage over the diode or diodes in the transmit branch.